1. Field of the Invention
The present invention relates to an optical pickup, and more particularly to an optical pickup in which a plate provided with a photodiode is bonded to an optical bench by an adhesive agent.
2. Description of the Related Art
Conventionally, an optical pickup having the structure shown in FIG. 3 has been known (e.g., refer to JP-A-2002-342947). In this optical pickup, an upper surface 12 of an optical bench 1 and edge portions of a plate 2, which is disposed on an upper portion of the optical bench 1 while maintaining a minute gap α between the plate 2 and the upper surface 12 of the optical bench 1, are bonded together through an adhesive agent layer 3. The plate 2 is provided with a photodiode 22 mounted on a flexible printed circuit board (FPC) 21, while the optical bench 1 is provided with a collimator lens 23, an objective lens 24, a semiconductor laser 25, a half mirror 26, and the like. Laser light from the semiconductor laser 25 is projected onto a disk D through the half mirror 26, the collimator lens 23, and as the objective lens 24, and as its reflected light is received by the photodiode 22, the disk D is optically scanned.
In this optical pickup, the operation of bonding the plate 2 to the optical bench through the adhesive agent layer 3 is conventionally carried out by the following procedure. Namely, in this operation, as shown by the phantom lines in FIG. 3, in a state in which the plate 2 is held by a positioning arm 100, the plate 2 is brought close to the optical bench 1 at a predetermined position by using the positioning arm 100. Subsequently, the photodiode 22 is positioned with respect to an optical axis P with high accuracy by effecting such as the positional adjustment of the plate 2 in three-axis directions which are perpendicular to each other. After such highly accurate positioning, an adhesive agent is injected to predetermined portions between the edge portions of the plate 2 and the upper surface 12 of the optical bench 1 and is allowed to cure.
As another conventional example, a fixing base provided with a photodetector is inserted in one hole formed in the optical bench, and the photodetector is positionally adjusted by pushing in or pulling out the fixing base in that hole. Subsequently, as a means of fixing that fixing base to the optical bench, instead of the method in which positioning screws are used a means is adopted wherein an adhesive agent is injected into the hole formed in the fixing base to bond the fixing base to the optical bench (e.g., refer to JP-UM-A-61-140126).
In addition, as shown in FIG. 4, it has been practiced to bond a plurality of peripheral portions of the plate 2 to the upper surface 12 of the optical bench 1 in spots by means of the adhesive agent layer 3, or to form recesses in a plurality of peripheral portions of the plate 2 and inject the adhesive agent into the recesses to form an adhesive agent layer.
However, the conventional example (JP-A-2002-342947) explained with reference to FIG. 3 has encountered the following problem. Namely, in this conventional example, the adhesive agent layer 3 is formed by allowing the adhesive agent injected between edge portions o the plate 2 and the upper surface 12 of the optical bench 1 to cure. For this reason, even if the amount of the adhesive agent injected is controlled so as to be fixed, the range of the spread of the adhesive agent is difficult to become uniform for each injected portion due to the flow of the injected adhesive agent. As a result, relatively large variations occur in the bonding area of the plate 2 and the bonding area of the upper surface 12 of the optical bench 1 with respect to the adhesive agent layer 3 at each of the plurality of portions formed through the curing of the adhesive agent. In addition, the adhesive agent forms the adhesive agent layer 3 as the adhesive agent cures by being accompanied with shrinkage. If there are thus large variations in the bonding areas of the plate 2 and the upper surface 12 of the optical bench 1 with respect to the adhesive agent layer 3 at the plurality of portions formed through the cure shrinkage of the adhesive agent, even if an attempt is made to position the photodiode 22 with respect to the optical axis P with high accuracy by using the positioning arm 100 described with reference to FIG. 3, a situation can occur in which the photodiode 22 becomes positionally offset from such a positioned point due to the cure shrinkage of the adhesive agent. Hence, there has been a problem in that the positioning accuracy of the photodiode 22 by the use of the positioning arm 100 is impaired by the bonding using the adhesive agent layer 3, which exerts an adverse effect on the reading performance based on the photodiode 22.
This problem similarly has occurred in cases where, as shown in FIG. 4, a plurality of peripheral portions of the plate 2 are bonded to the upper surface 12 of the optical bench 1 in spots by means of the adhesive agent layer 3, or in cases where recesses are formed in a plurality of peripheral portions of the plate 2, and the adhesive agent is injected into the recesses to form the adhesive agent layer.
Further, in the other conventional example (JP-UM-A-61-140126), since the technique of using the adhesive agent instead of the positioning screws is merely proposed in the case of fixing the fixing base to the optical bench, the technique based on this conventional example is not a technique which can overcome the above-described problem that the variations in the bonding areas using the adhesive agent layer impair the positioning accuracy of the photodiode. In addition, with this conventional example, since a method is adopted in which the fixing base is fixed to the optical bench by injecting the adhesive agent into the single hole formed in the fixing base, it is impossible to avoid a situation in which the fixing base is drawn toward the hole side due to the cure shrinkage of the adhesive agent, thereby impairing the initial positioning accuracy.